chore(cleanup): purge /command residuals — fakeEngine, canon golden, openapi

Follow-up tidy after the cross-service /command removal (#73):

- Rename the router test double dummyExecutor -> fakeEngine (and the
  newExecutor / setupRouterExecutor helpers -> newFakeEngine /
  setupRouterEngine): it implements handler.Engine now, "executor" was a
  leftover of the removed adapter. Test-only.

- Regenerate the ui/core canon signing golden onto user.games.order
  (request_user_games_command.json -> request_user_games_order.json, fresh
  canonical bytes + Ed25519 signature) and drop the last
  user.games.command references from the Go/TS tests and docs.

- Align game openapi: CommandRequest.cmd no longer carries minItems: 1. It
  is now used only by PUT /api/v1/order, which accepts an empty batch
  (clearing the player's stored order, equivalent to removing every
  command); the contract test freezes the empty-allowed shape.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ui/frontend/tests/wasm-core-canon-parity.test.ts

@@ -79,7 +79,7 @@ beforeAll(async () => {
 describe("WasmCore canon parity with gateway fixtures", () => {
 	test.each([
 		"request_user_account_get.json",
-		"request_user_games_command.json",
+		"request_user_games_order.json",
 		"request_lobby_my_games_list.json",
 	])("%s — canonical bytes byte-for-byte equal", (name) => {
 		const fixture = readJson<RequestFixture>(name);